This invention relates generally to improved, high permeability, multi-layer woven members for use in papermaking machines. In the preferred embodiments, the members are employed to convey fibrous webs through the dryer section of a papermaking machine. Most preferably the woven members of this invention are carriers, or base fabrics for a resinous, embossing layer cast thereon. In accordance with this invention the woven members employ a unique woven structure to achieve numerous benefits.
Multi-layer fabrics are widely known in the papermaking art and are employed in various sections of a papermaking machine. One such multi-layer structure, which preferably is employed in the dryer section of a papermaking machine, is disclosed in Gaisser U.S. Pat. No. 5,114,777. The woven fabric disclosed in Gaisser is a two layer structure including pairs of vertically stacked warp yarns that are spaced apart from each other in the cross-machine-direction and are held together in their vertically stacked relationship by a plurality of single weft yarns spaced along the machine direction of the fabric. Although this fabric has been used commercially in dryer fabrics, improvements are desired in material costs, stability, seam strength and drainage or water extraction from fibrous webs without retention of excess water in the fabric.
Other multilayer papermaking fabrics employing warp binder yarns and being usable in various sections of a papermaking machine are disclosed in the prior art. For example, Stelljes, Jr. et al. U.S. Pat. No. 5,840,411 discloses a number of different embodiments of two-layer fabrics wherein each layer includes interwoven weft and warp yarns. These layers are connected together by tie warns, which can be separate warp or weft yarns that are independent from the weave pattern in either layer, or warp or weft yarns that also contribute to the weave pattern in one or both of the layers. Also, these tie yarns are described as possibly being of a reduced diameter to minimize blockage of open areas through the fabric. The structures disclosed in this patent employ two distinct layers, each including interwoven single warp and single weft yarns and are not directed to structures employing a plurality of paired warp yarns in the weave construction.
U.S. Pat. No. 4,995,529, issued to Kositzke, discloses a multilayer film including upper and lower, self-sustaining layers, each including interwoven machine direction and cross machine direction yarns. Although this patent discloses the vertical aligning of machine direction and cross machine direction yarns in each layer, it does not relate in any way to structures including paired machine direction yarns in either layer.
U.S. Pat. No. 5,454,405, issued to Hawes, discloses a triple layer papermaking fabric including top and bottom weft yarn layers interconnected by top warp yarns that form part of a warp yarn system with underlying warp yarns. The underlying warp yarns are not employed to bind the top and bottom weft yarn layers together. This patent does not disclose a fabric employing paired warp yarns transversely aligned in a single layer.
U.S. Pat. No. 5,219,004, issued to Chiu, discloses multilayer structures employing bottom warp binder yarns. In one disclosed embodiment these bottom warp yarns are included in pairs, with each pair being located in a region underlying the open area between two, single top warp yarns. These disclosed fabrics are described as being forming fabrics usable in the wet end of a papermaking machine and are neither designed nor intended to be employed as a dryer fabric in a dryer section of a papermaking a machine. In particular, the fabrics disclosed in the Chiu ""004 patent have a substantially lower open area than is desired or needed in dryer fabrics of papermaking machines. In fact, the top ply or layer of the Chiu structure preferably has twice as many shute yarns as in the bottom side, with the shute yarns on the bottom side underlying the open area between the shute yarns in the top ply. This impedes the flow of water through the fabric, and would be undesirable for use in dryer fabrics; particularly through air dryer (TAD) fabrics.
Other types of fabrics employing partner yarns are disclosed in Fitzka et al. U.S. Pat. No. 5,092,372. In these fabrics extra partner yarns are added to a top ply to vary the number of fiber supporting points in the fabric.
Other multi-layer fabrics for use in papermaking machines are disclosed in U.S. Pat. No. 4,314,589 (Buchanan et al.); U.S. Pat. No. 4,501,303 (Osterberg); U.S. Pat. No. 4,705,601 (Chiu); U.S. Pat. No. 4,729,412 (Bugge); U.S. Pat. No. 4,832,090 (Krenkel et al.); U.S. Pat. No. 4,945,952 (Vohringer); U.S. Pat. No. 5,054,525 (Vohringer); U.S. Pat. No. 5,151,316 (Durkin et al.) and U.S. Pat. No. 5,152,326 (Vohringer).
Applicants believe that a need exists for improved, high permeability, multi-layer woven members in papermaking machines; preferably in dryer fabrics, and more particularly in through air dryer (TAD) fabrics, which have excellent seam strength, excellent stability, high drainage and lower water retention properties than prior art dryer fabric structures and that are capable of supporting, or receiving a cast, resinous, embossing layer employing less resinous material than prior art structures. It is to such woven members and composite dryer fabrics employing same that the present invention is directed.
The above and other objects of this invention are achieved in a multi-layer woven fabric, which, when flat woven, includes a top layer with a self-sustaining weave construction including top warp yarns extending in the machine direction of fabric movement through a section of a papermaking machine interwoven with top weft or shute yarns extending in the cross-machine-direction of a papermaking machine, most preferably but not necessarily in a plain weave pattern for engaging and/or supporting a fibrous web in a papermaking machine. A bottom layer includes bottom pairs of warp yarns extending in the machine direction of fabric movement through a section of a papermaking machine, and bottom weft, or shute yarns extending in a transverse, or cross-machine-direction substantially normal to said machine direction.
Most preferably, the fabrics of this invention are formed in a flat weaving process, resulting in the warp yarns being disposed in the machine direction of fabric movement through a section of a papermaking machine and the weft yarns being disposed transversely to the warp yarns, i.e., in the cross-machine-direction of the papermaking machine. Although it may be difficult to form the fabrics of this invention in an endless weaving process, if such a process is employed than the weft yarns will be disposed in the machine direction of fabric movement through a section of a papermaking machine and the warp yarns will be disposed transversely to the weft yarns, i.e., in the cross-machine-direction of the papermaking machine. In accordance with the broadest aspects of this invention the fabrics may be formed by either a flat weaving process or an endless weaving process; although the flat weaving process is preferred.
Reference throughout this application, including the claims, to fabrics having yarns being in the xe2x80x9cmachine directionxe2x80x9d refers to the direction of such yarns on a papermaking machine; not on a loom employed to manufacture the fabrics. Similarly, reference throughout this application to fabrics having yarns being in the xe2x80x9ccross-machine-directionxe2x80x9d refers to the direction of such yarns on a papermaking machine; not on a loom employed to manufacture the fabrics.
For ease of discussion, throughout the remainder of this application flat woven fabrics of this invention will be described, wherein the first and second layers each include warp yarns extending in the machine direction. However, as stated above, in endless woven fabrics of this invention weft yarns will extend in the machine direction.
In preferred embodiments of the invention the bottom warp yarns are paired together to form a plurality of spaced-apart pairs of contiguous warp yarns. The top warp yarns preferably are single warp yarns that are spaced apart in the cross-machine direction a sufficient distance to provide a desired projected open area through the top, paper side layer; preferably, but not limited to at least 25% for permitting the unimpeded passage of air therethrough when the fabric is employed in the dryer section of a papermaking machine.
The bottom pairs of warp yarns are spaced-apart in the cross-machine-direction from adjacent pairs of warp yarns so that each pair substantially vertically underlies a single top warp yarn, and the plurality of bottom weft yarns are spaced-apart in the machine direction so that each bottom weft yarn substantially vertically underlies a top weft yarn; the number of bottom pairs of warp yarns corresponding to the number of individual top warp yarns and the number of bottom weft yarns corresponding to the number of top weft yarns. The bottom warp and weft yarns form a bottom, wear side layer that engages vacuum boxes, drive rolls, and the like. This bottom layer, like the top layer, preferably has a projected open area of at least 25%.
The warp yarns in each pair of warp yarns preferably contact each other along a tangent line, but in some cases may be slightly spaced apart due to slight movement occurring during the weaving operation, or in use of the fabric. However, in all cases the space between adjacent pairs of warp yarns is substantially larger than any spacing that might exist between the yarns in each pair.
At least one warp yarn in at least some of the bottom pairs of warp yarns constitute binder warp yarns that interweave the bottom weft yarns with one another and with the weft yarns in the top layer. In a preferred embodiment of the invention each warp yarn in each respective bottom pair of warp yarns interweaves either with the same or with a different top weft yarn of the top layer; most preferably with a different top weft yarn of said top layer. However, it is within the scope of this invention to employ only one of the bottom warp yarns in all or some of the bottom pairs of warp yarns as a binder yarn to engage a top weft yarn of the top layer. In this latter embodiment, the binder yarn in each pair of bottom warp yarns can have a smaller diameter than its paired warp yarn, which does not move up to the top layer to provide a binding function. This results in less obstruction of the open area than would be the case if the binder yarn were of the same diameter as its paired bottom warp yarn.
Unlike prior art structures, neither the bottom weft yarns nor the bottom pairs of warp yarns excessively block the open areas provided in the top layer since they are in substantial vertical alignment with the top weft yarns and top warp yarns, respectively. This unobstructed condition exists except in the very limited regions in which bottom warp yarns function as binder yarns to interweave with top weft yarns in the upper layer, with each bottom warp binder yarn being closely adjacent to its overlying top warp yarn.
Reference throughout this application to xe2x80x9csubstantiallyxe2x80x9d or xe2x80x9csubstantialxe2x80x9d in describing the vertical alignment or vertical arrangement of the weft yarns in one layer with the weft yarns in the other layer, and the vertical alignment or vertical arrangement of a single or pair of warp yarns in one layer with a pair of warp yarns in the other layer, includes a relationship wherein the weft and warp yarns in one layer are in precise vertical alignment with the weft and warp yarns in the other layer, respectively, as well as an arrangement wherein a weft yarn and a single warp yarn or pair of warp yarns in one layer at least partially overlie (or partially underlie) a weft yarn and a pair of warp yarns in the other layer, respectively. For example, when a single warp yarn in one layer is in precise vertical alignment with a pair of warp yarns in the other layer the central axis of the single warp yarn is in generally vertical alignment with the line of contact, or contiguous line or very narrow region between the warp yarns in the pair of warp yarns. However, a single warp yarn, or optionally a pair of warp yarns in one layer is xe2x80x9csubstantiallyxe2x80x9d in vertical alignment with a pair of warp yarns in the other layer even if it is offset in a lateral direction to only partially overlap with the pair of warp yarns in the other layer. However, there does need to be sufficient overlap between the warp yarns in the respective layers to maintain the two layers separated when such layers are bound together by warp binder yarns.
Thus, unlike prior art constructions employing precisely vertically aligned, single warp yarns in opposed first and second layers, it is much easier to maintain the desired substantial vertical alignment in the fabrics of this invention, wherein the warp yarns in at least one layer are disposed in pairs.
Similarly, when a single weft yarn in one layer is in precise vertical alignment with a weft yarn in the other layer the central axes of the respective weft yarns are in general vertical alignment. However, a weft yarn in one layer is in substantial vertical alignment with a weft yarn in the other layer even if the axes are laterally offset; provided that there is some overlap of these weft yarns.
In view of the substantially vertically stacked relationship of the top and bottom weft yarns, on the one hand, and each top warp yarn with a respective pair of bottom warp yarns, on the other hand, vertical, unimpeded passages are provided through each open fabric area bound by a pair of spaced-apart adjacent top warp yarns and spaced-apart adjacent top weft yarns and underlying adjacent pairs of warp yarns and spaced-apart adjacent bottom weft yarns.
In other words, these latter open areas, which are essentially quadrilateral in plan view, are not obstructed in a vertical direction by either the bottom weft yarns, which are stacked substantially vertically beneath the top weft yarns, or by the pairs of bottom warp yarns, which are stacked substantially vertically beneath the top warp yarns, except where a bottom warp yarn functions as a binder yarn by interweaving with a top weft yarn to bind the top woven layer and the bottom weft yarns together into a self-sustaining, multi-layer fabric construction. However, even where bottom warp binder yarns interweave with a weft yarn in the top, layer, they do so in a region closely adjacent to, or contiguous with their substantially vertically overlying top warp yarn so as not to excessively block the open area vertically through the member. Moreover, the number of these binder locations is minimal so as not to adversely affect fluid flow through the woven fabric.
Preferably, the projected open area in both the paper side layer and the wear side layer is at least 25%; more preferably at least 30%; still more preferably close to 35%. In fact, in the most preferred embodiments of this invention, when the papermaking fabric is a through air dryer (TAD) fabric, the projected open area in the paper side layer, prior to including a cast, resinous embossing layer thereon, is at least 35 percent, more preferably in excess of 40% and in some constructions in excess of 50%. The desired projected open area in said paper side layer and wear side layer is provided by the selection of yarn diameters and yarn spacing, consistent with obtaining other required properties in the fabric, e.g., stability and stiffness.
In accordance with the most preferred embodiments of this invention the projected open area in one of the layers is different than the projected open area in the other layer. This results from the spaced apart warp yarns in one layer having a different diameter, or transverse dimension parallel to the plane of the fabric than the transverse dimension of substantially vertically aligned pairs of warp yarns in the other layer. In the preferred embodiment of this invention the paper side layer includes spaced-apart single warp yarns substantially vertically overlying paired warp yarns in the wear side layer; thereby resulting in a fabric having a higher projected open area in the paper side layer than in the wear side layer. In fact, the open area in the vertical direction through the fabric is generally funnel-shaped; being larger in the paper side layer and smaller in the wear side layer. This difference in projected open area may provide advantages in controlling air flow through the fabric. In addition, providing a lower projected open area in one of the layers may permit the use of less resin in casting an embossing layer on the fabric to achieve a desired porosity through the fabric. In other words, since the projected open area in one of the layers is less than the projected open area in the other layer, less resin will be required in the lower projected open area region to achieve a desired air permeability, as compared to the amount of resin required to achieve that same level of air permeability in a region of the fabric having a higher projected open area prior to casting. The use of less resin results in a desirable reduction in material costs.
In a preferred form of this invention, the above described, multilayer members of this invention are carrier fabrics for receiving a resinous embossing layer cast thereon, such as a layer of the type disclosed in FIGS. 2 and 3 of the aforementioned Gaisser ""777 patent. The subject matter of the Gaisser ""777 patent is fully incorporated herein by reference, it being understood that the specific resinous embossing layer is a structure well-known to those skilled in the art and does not constitute a separate and independent invention of the present applicants. In fact, representative constructions employing a cast, resinous framework on a woven carrier fabric are disclosed in U.S. Pat. No. 4,514,345, issued to Johnson et al.; U.S. Pat. No. 4,528,239, issued to Trokhan; U.S. Pat. No. 4,529,480, issued to Trokhan and U.S. Pat. No. 4,637,859, issued to Trokan. The subject matter in these latter four patents is fully incorporated herein by reference.